how fast?

first you will not even break 17 for the 5k this year.

you'r way to ambitious. the rapid improvement you see now is normal, you'll stale sooner or later though

I started up a month ago after an 18-month layoff. I did a 18:55 5K.

Yesterday I went out for a 12-mile run and passed the 3-mile mark at around 18:50. (so 5K in about 19:35)

the jump down to 18:30 was probably just the normal jump from getting back into shape. to make another jump from 18:30 to 15:30 would be harder by great magnitudes. 15:30 is not for this year, MAYBE for next if you train all year.

Sorry I didn't finish. I meant to also say that your fitness should come back faster than you think if you haven't put on too much weight and have been semi-active.

Your PRs indicate that you are probably a low-to-mid 14:00 guy, so to be within 60-90s of your PR after a summer of training sounds very reasonable.

Well... I guess we will see. I haven't been active (due to injury... long story)... but I do weigh the same that I did in college, and I had rapid improvement back then (didn't run in high school... just college).

I ran 30 miles a week during track and maybe hit 50 two or three times during the cross season (which I only ran for 2 seasons...usually around 40 miles a week).... So I figured if I can increase my mileage up to about 50 a week, I might have a shot to hit 15:30 in September... Next 5k is in a month, and I plan on running in the low 17's.... but I guess we will see.